Piezoelectric transducers are described in principle, for example in DE-B-34 25 992 (U.S. Pat. No. 4,721,106). The use of a coupling medium for coupling ultrasonic shock waves to a patient's body with such transducers is known.
Although transducers have been successfully used in therapy, the structural dimensions thereof need to be very large, if the energy density at the focus is to be sufficient for the disintegration of a concretion which is to be destroyed.
Although the energy densities that can be produced by means of piezoelectric materials are very high, only a very small proportion of the energy produced is, in practice, passed into the coupling medium, which may be water or oil, since the sound-producing ceramic and the water or oil differ very greatly from one another acoustically.